There are several currently known types of optical fiber cable structure: in particular there are optical fiber cables in which the optical fibers are left loose or disposed in ribbons inside a central tube, or optical fiber cables in which the optical fibers are disposed in the helical grooves of a central grooved rod. Other structures exist which are not described in detail herein, but to which the present invention may also be applied.
In all optical fiber cable structures, it is necessary to provide mechanical reinforcing members so as to avoid subjecting the optical fibers to large forces, in particular traction and bending forces on installing the cable in a duct or when the cable is suspended, which is becoming increasingly common for optical fiber cables. Mechanical reinforcing members also serve to take up the effects of variations in the lengths of the various elements of the cable other than the optical fibers (the central strength member, e.g. in the form of a tube or a grooved rod, and the protective jackets) when the cable is subjected to large temperature variations. Therefore, the reinforcing members must have a thermal expansion coefficient that is lower than those of the remaining portions of the cable, and a Young's modulus that is higher than those of the remaining portions of the cable (other than the optical fibers which, in particular, have a thermal expansion coefficient that is much lower than those of the other elements of the cable).
The reinforcing members may be metallic or dielectric. Reinforcing members that are non-metallic, i.e. that are not electrically conductive, are preferred so as to avoid the risks of damage due to lightning, and the problems of induced voltages. Therefore, currently used reinforcing members are dielectric, i.e. made of non-conductive materials.
Plastics materials that are reinforced with reinforcing fibers are particularly suitable for such reinforcing members. Such materials are commonly referred to as "Fiber-Reinforced Plastics" (FRP) and they comprise reinforcing fibers, e.g. fibers made of aramid, e.g sold under the Kevlar trademark, of polyester, or of glass, embedded in a resin, e.g. such as an epoxy resin or a polyester resin. The properties of these materials make them particularly suitable for mechanically reinforcing optical fiber cables.
Optical fiber cables using FRP reinforcing members are made using two different methods.
In a first method, the result of which is shown in FIG. 3B of U.S. Pat. No. 4,770,489, for example, which figure shows an optical core assembly, e.g. constituted by a tube containing loose optical fibers, surrounded by a plurality of reinforcing members made of an FRP material, each of the reinforcing members is manufactured on its own from reinforcing fiber strands coated with resin, and the members are then twisted onto the optical core assembly, i.e. they are assembled helically therearound, during a distinct subsequent twisting step. The structure of the resulting cable is advantageous because it makes the cable relatively flexible, and imparts the desired mechanical strength properties to it.
Unfortunately, making that type of structure poses certain problems.
Firstly, making the reinforcing members on their own is expensive.
Furthermore,, in addition to the step of manufacturing the reinforcing members entirely, a subsequent twisting step is required, and this makes manufacturing the cable lengthy and complex, and therefore expensive.
In a second method, the result of which is shown in FIG. 2A of U.S. Pat. No. 4,770,489, the above-mentioned problems can be solved. A tube is made of an FRP material around the optical core assembly, e.g. by means of an operation referred to as "pultrusion" (as opposed to "extrusion"), which consists, on a production line, in bringing reinforcing fiber strands to the vicinity of the optical core assembly, and then in causing the resulting assembly to advance by pulling it successively through a resin-coating bath, a calibration die for imparting the desired cross-section to the cable, and finally apparatus for polymerizing the resin.
The pultrusion method enabling an FRP tube to be made around an arbitrary element is described in detail in Patent Application EP-0 284 667 (in that document, the tube is made directly around an optical fiber).
The second method is advantageous because it is less expensive to implement than the above-described first method. The reinforcing member is manufactured in a single step directly around the optical core assembly. Furthermore, the resulting structure exhibits the desired mechanical characteristics.
Unfortunately, that structure is very rigid, so that installing it in underground ducts or suspending it from pylons poses problems.